The determination of a battery's state of charge (SOC) and state of health (SOH) are important for the power management of any system that relies on batteries as a power source, e.g. cell phones, laptops, electric vehicles, grid level storage and the like. Despite its fundamental importance in battery systems, SOC and SOH measurements are not straightforward and remain an area of active research.
To date, the state of charge of lithium-ion batteries is estimated either by cell voltage, coulomb counting, or a combination of both methods. Voltage-based SOC measurements are relatively imprecise because voltage is somewhat insensitive to SOC in commercial lithium-ion chemistries. Voltage is also dependent on a number of factors such as temperature and electric current. Coulomb counting is a technique which relies on measuring the amount of charge entering and leaving a battery to determine the battery's state of charge. This technique builds up errors over time and is mathematically intensive. This type of measurement scheme requires periodic recalibration through a controlled full charge and discharge cycle to maintain accuracy and the current measurement results in a small power loss. Furthermore, this method cannot account for SOC change due to self-discharge.
The state of health of a battery cell is typically measured by fully charging and then fully discharging the battery cell using a controlled testing protocol in a controlled environment to determine its remaining charge storage capacity. This technique is unsuitable for consumer applications as it is difficult to create suitable conditions for a controlled capacity measurement outside of a laboratory setting. In addition, a controlled capacity measurement requires the battery cell to be offline for the duration of the measurement, which generally takes many hours to complete. For these reasons, determination of a battery cell's SOH is an active area of research. Typical approaches attempt to correlate conventionally measured battery parameters (voltage, current, temperature, SOC, etc.) to SOH. Another actively investigated approach is to correlate measurements of AC impedance to SOH, which requires the addition of an impedance measurement system. Both approaches rely on complicated models and generally exhibit limited accuracy in determining SOH. Improved systems and methods for determining SOC/SOH are desirable.